Communications equipment, computers, military targeting systems, home stereo amplifiers, televisions, and other electronic devices are increasingly characterized by small electronic contacts and miniature components which are very vulnerable to interference or damage from stray electrical energy surges of both the transient and continuous type carried by alternating current (AC) power lines. Unpredictable variations in power line voltage, as well as continuous noise thereon, changes the operating range of the equipment and can severely damage and/or destroy electronic devices. Moreover, these electronic devices can be very expensive to repair and replace and, therefore, require cost effective transient and continuous type noise protection.
There are many sources which can cause harmful electrical energy surges. One source is radio frequency (RF) interference that can be coupled to the power lines from a multitude of sources. The power lines act as large antennas that can be stretched out over several miles and which can have significant radio frequency noise coupled thereto from such sources as radio broadcast antennas. Another source of the harmful radio frequency energy is from the equipment to be protected itself, such as computers. Today, the Federal Communications Commission (FCC) has elaborate test procedures to prevent computers from being marketed that cause severe noise problems. Nevertheless, older computers still exist which emit significant amounts of radio frequency interference. Another harmful source is conductive noise, which is generated by equipment connected to the power lines and which is conducted along the power lines to the equipment to be protected. Still another source of harmful electrical energy is lightning. Lightning is a very complex electromagnetic energy source having potentials estimated at from 5 million to 20 million volts and currents reaching thousands of amperes (amp). A lightning strike generally contains a series of pulses, each having a duration of from one nanosecond (10.sup.-9 s) to several milliseconds (10.sup.-3 s). A typical "8/20" lightning pulse lasts for a period of 40 milliseconds and has a peak current of 20,000 amperes, which is reached in 8 microseconds.
In order to provide protection to circuitry against the foregoing sources of harmful electrical energy, a transient voltage surge must be clamped to a safe level, and the current must be adequately dissipated. There are generally three types of surge suppression devices or regulator devices commonly employed in low cost power line surge suppression units: (a) metal oxide varistors (MOV); (b) silicon avalanche diodes; and (c) gas discharge tubes (GDT). These devices have different operating characteristics and ratings and different failure, or "wear out," characteristics. Consequently, oftentimes, these devices are aggregated together to take advantage of the desirable characteristics of each device.
In surge suppression units using an MOV, the MOV clamping voltage must be above the peak of the highest expected line voltage. For nominal 120 volts alternating current (AC), with a .+-.10% variation tolerance, this means that the line voltage could be 132 volts AC. The peak voltage amplitude would be this number multiplied times the square root of 2 (i.e., 1.414), or 186 volts. Allowing for a 20% tolerance variation of the MOV, this brings the MOV rate of the clamping voltage to 225 volts. A 1 inch diameter MOV, rated at 70 joules (J), can absorb as much as 6,500 amperes of surge current before being destroyed. However, the high currents, along with the internal resistance of the MOV, allows the clamping voltage level to exceed its rating, which is usually at 10 amperes. Real clamping levels at 3,000 amperes can exceed 400 volts. The 400 volts clamping level is usually enough to damage most electronic equipment.
As mentioned, another commonly used component is the silicon avalanche diode, which is really a pair of back-to-back conventional zener diodes. This device has the advantage of a 5% tolerance and a lower impedance than the MOV. Accordingly, a lower clamping voltage can be specified without fear of coming too close to the 186 volts line peak. A 195 volts diode with 5% variation tolerance is usually specified. However, the current handling capability of the diode is limited to several hundred amperes, and a 3,000 ampere transient would certainly destroy it. Even so, it does have the advantage of maintaining its clamping voltage throughout its current range.
Furthermore, another commonly used component is the gas discharge tube (GDT). Such a device operates very slowing and ignores fast rise time transients. It also has the disturbing characteristic of remaining shorted until all current has been removed. Thus, it typically turns a transient surge into a total loss of power and a tripped circuit breaker.
Although meritorious to an extent, all of the previously described devices, combinations thereof, and other known arrangements in the industry have not proven to be entirely satisfactory. Accordingly, there is a heretofore unaddressed need in the industry for an improved transient voltage surge suppressor which is devoid of the drawbacks of the known arrangements, which is inexpensive, and which is requires little space for implementation.